Whole human blood includes at least three types of specialized cells. These are red blood cells, white blood cells, and platelets. All of these cells are suspended in plasma, a complex aqueous solution of proteins and other chemicals.
Until relatively recently, blood transfusions have been given using whole blood. There is, however, growing acceptance within the medical profession for transfusing only those blood components required by a particular patient instead of using a transfusion of whole blood. Transfusing only those blood components necessary preserves the available supply of blood, and in many cases, is better for the patient. Before blood component transfusions can be widely employed, however, satisfactory blood separation techniques and apparatus must evolve.
Plasmapheresis is the process of taking whole blood from a donor and separating the whole blood into a plasma component and a non-plasma component under conditions whereby the plasma component is retained and non-plasma component is returned to the donor.
Thrombocytapheresis is similar, except that whole blood is separated into a platelet component and non-platelet component with the platelet component retained or "harvested" and the non-platelet component returned to the donor.
A particularly useful device for the collection of blood cell components is the Haemonetics.RTM.30 Cell Separator Blood Processor manufactured by Haemonetics Corporation, Braintree, Mass. (hereinafter the Model 30). The Model 30 utilizes a conically-shaped centrifuge bowl similar to the bowl described in U.S. Pat. No. 3,145,713, FIG. 6, now called the Latham Bowl. The bowl is held in a chuck which is attached to a spindle and driven by a motor. The bowl consists of a rotor portion wherein blood component is separated and a stator portion consisting of an input and output port. One side of the input port is connected through a first peristaltic pump to a source of whole blood from a donor and the other side is in fluid communication with a fractionation volume in the rotor. Anticoagulant is mixed with the whole blood prior to entry into the centrifuge bowl.
The rotor is rotated at a fixed speed and various blood fractions are collected at the output port and directed into appropriate containers by diverting the flow through tubing in accordance with the setting of three-way clamp/switches.
Fractionation within the centrifuge is determined by the relative densities of the different cell components being separated and collected. The various cell fractions pass through the outlet port of the centrifuge bowl by progressive displacement from the lower portion of the bowl. The operator is trained to visually observe and assess the boundaries or demarcation lines of different component layers as they approach the outlet port of the centrifuge bowl. When the desired fraction has exited the bowl, the centrifuge is stopped. The flow is then reversed and the uncollected cells, such as packed red blood cells (RBC) are returned to the donor.
As a practical matter, however, the boundary between cell layers separated by centrifugation alone is often indistinct.
Aisner et al. in a paper published in Transfusion Sep-Oct 1976 entitled "A Standard Technique for Efficient Platelet and Leukocyte Collection Using the Model 30 Blood Processor" graphically illustrates the problems associated with a visual determination of cell fractionation in the plot of FIG. 1 page 438 (reproduced on a different scale in FIG. 2 on this patent application). The results of the experiments reported in the Aisner et al. paper show that considerable overlap exists between the platelet fractionation, the white blood cell (WBC) fractionation and the red blood cell (RBC) fractionation. The reason for this overlap is that a particular cell population will be distributed over a range of densities. Thus, any attempt to separate cells solely by density, as in a centrifuge process such as the Model 30, is bound to result in a certain degree of overlap.
What this means in practice is that cross-contamination of the isolated cells invariably results from the overlap in the range of cell densities. Cross-contamination limits the degree of purity of the isolated cell fractions and makes recognition very difficult and inaccurate, and non-repeatable by visual observation of an operator or other sensory means.
The result is that a compromise is made in present practice. If RBC-free platelets are being harvested, the process is stopped when the fractionation line from the centrifuge bowl to the platelet bag turns a light pink indicating the presence of RBC. This greatly reduces the platelet yield for each cycle through the bowl but insures a substantially RBC-free collection of platelets.
Alternatively, a high platelet yield can be achieved at the expense of an additional centrifuge process by continuing collection of platelets for a predetermined time interval after the observance of the commencement of a pink fraction. This is called the "red cell technique". In the Model 30, the time interval used is about 60-90 seconds. This increases the platelet yield substantially, but requires a subsequent operation in which the collection bag with platelets and a substantial quantity of RBC and WBC is removed from the system and centrifuged at low speeds (150 g) for about 7 minutes, whereupon the supernatant containing the platelets may be expressed, leaving the RBC and WBC in the bag.
In addition to the added time involved in this procedure, the donor is usually not available after this procedure, so the RBC and WBC can no longer be returned to the donor. Additionally, about 10%-25% of the platelets are lost in the process; and the added centrifuge process involves entry into the platelet bag which, while meant to be aseptic, introduces an added risk of contamination.
A known technique for achieving separation of light particles from heavier particles is the process of elutriation. Elutriation achieves separation by causing fluid flow past the particles. This principle has been applied to a pheresis process by Cullis et al. in U.S. Pat. No. 4,187,979 wherein whole blood is collected from a donor in a whole blood bag and then pumped to a first separation chamber mounted on the rotor of a centrifuge. The first separation chamber consists of a diamond-shaped bag with two sets of inlet and outlet ports. Each inlet port in a set is located diagonally opposite its outlet port. Blood fluid, rich in plasma and low in WBC and platelets, is allowed to flow from a lower inlet port to an upper outlet port transverse to the flow of whole blood across the separation chamber from a side corner inlet port to a side corner outlet port. According to Cullis et al. (col 7, lines 28-35) "In this way, the plasma flow crossing the whole blood flow will elute the white blood cells and platelets from the whole blood and at the same time, the plasma will wash the red blood cells." While this elutriation process is occurring in the first separation chamber, the WBC and platelets may be separated by centrifugation and sedimentation in second and third separtion chambers.
The Cullis et al. system, requiring as it does a four port primary separation chamber and transverse flow of fluid for elutriation, is not compatible with existing hardware/software, such as the Latham bowl, which has only one inout and output port. Also, Cullis et al. is principally directed to the problem of harvesting only red cells whereas an equally important need exists for a method and apparatus of increasing cell-free platelet yields in existing bowl-type centrifuges without destroying or contaminating RBC and, perhaps more significantly, WBC, which are returned to the donor. Additionally, the collection bags for harvesting components from the primary separation chamber are all mounted on the centrifuge rotor making access difficult and, incidentally, subjecting components in such bags to additional unnecessary centrifugal forces.